The present invention relates to conditioning of tobacco, and more particularly to improvements in a method and apparatus for reducing the moisture content of tobacco. Still more particularly, the invention relates to a method and apparatus for reducing the moisture content of tobacco particles which are transported in the form of a continuous stream.
It is necessary to reduce the moisture content of tobacco leaves or shreds prior to conversion of dried tobacco into the fillers of cigarettes, cigars or other rod-shaped smokers' products. As a rule, the final moisture content of tobacco should coincide with or deviate only negligibly from a preselected final moisture content because any appreciable deviation from such preselected moisture content affects the quality of ultimate product and/or involves additional expenses to the manufacturer. For example, the weight of a cigarette must match or exceed a prescribed minimum weight and, therefore, if the particles of tobacco which form the fillers of cigarettes are too dry, the quantity of tobacco in such cigarettes must be increased with resulting losses to the manufacturer. Such losses can be avoided if the shreads which are to be converted into the filler of a cigarette rod are dried in such a way that their moisture content equals or closely approximates a preselected final moisture content.
U.S. Pat. No. 3,372,488 discloses a tobacco drying apparatus wherein a continuous stream of moist tobacco is conveyed through a conditioning chamber which is defined by a rotary drum and wherein tobacco exchanges heat with a fluid. The quantity of heat which is supplied by the fluid varies in dependency on changes in the initial moisture content of tobacco. The fluid is usually heated air. A drawback of such apparatus is that it must be equipped with means for heating large quantities of air so as to be capable of properly drying tobacco having a relatively high or relatively low moisture content. When the initial moisture content of tobacco is low, the flow of heated air is throttled to prevent excessive drying of tobacco; on the other hand, the quantity of heated air which contacts the particles of tobacco in the rotating drum is increased when the initial moisture content of tobacco is higher. Such mode of operation necessitates the heating of excessive quantities of air. Moreover, certain types of tobacco, or tobacco particles which underwent a special preliminary treatment (e.g., tobacco containing large quantities of casing), should not be contacted with substantial quantities of air. In order to avoid undesirable reactions, such types of tobacco must be dried with relatively small quantities of air which, in accordance with presently known methods, results in relatively low output of the apparatus. The situation is similar when the heating fluid is conveyed countercurrent to the direction of movement of tobacco particles through the conditioning chamber. The apparatus must produce relatively large quantities of heated fluid because the initial moisture content and/or quantity of tobacco varies, often within an extremely wide range.